The present invention relates to a print control device and method for controlling the heat energy supplied to a thermal head of a printer apparatus that develops color on a medium in gradations according to the amount of thermal energy applied, or executes printing by image transfer using fusion transfer of an intervening thermal-transfer film or a sublimation transfer thereof.
Conventionally, as a method for controlling the heat energy to be applied onto a thermo-sensitive recording medium, it was common to use a method generally called xe2x80x9cheat history controlxe2x80x9d, in which the temperature of fixed-resistance heating elements on a thermal head was estimated based on past print history information to thereby control the amount of heat energy to be generated by the fixed-resistance elements on the thermal head.
This method is performed by making an estimation, therefore, since different heat emission conditions apply between printing executed in a cold region and that in a tropical region, and since the temperature on the surface of the paper medium also varies accordingly, there was a problem that control errors were easily generated. Accordingly, since the control was performed according to a calculation based on an estimation, it was difficult to achieve a high precision and stable print control.
There is another method in which an alloy such as Cr or Al or the like, which is a material that changes its resistance value depending on its temperature, is used as a heating element to construct the thermal head, and during the printing its temperature is measured so that the print history is not relied upon in order to perform the print control. However, even in this method the object of control is not the value of the heat energy generated by the heating member. Rather, the control is performed using the detected temperature data, so there was a problem that the color development density to be realized on the thermal recording medium could not be controlled accurately.
Further, it was also necessary to deal with an error in the print density caused because, as reflected in the fact that the color development characteristic of thermo-sensitive recording medium is called y-property, there is no linear proportional relationship between the supplied heat energy and the color development density.
Additionally, there was a problem that it was impossible to detect overheating of the thermal head even when damage was likely to occur as a result, which affected the reliability of the print control. For example, if heating control was executed in a state when the thermal head was not in contact with the surface of the paper medium, the temperature of the heating members of the thermal head rose to an abnormally high level and therefore the heating members became burnt and damaged.
An object of the present invention is to solve the above problems inherent in the conventional devices and methods and to provide a print control device and method capable of high precision and stable print control.
That is, according to the present invention, there is provided a print control device comprising: a thermal head provided with a set of minute heating members, each serving as both a heating element and a temperature detector, and a drive circuit for supplying an electric current to drive said heating members; a control circuit for effecting switching of the electric current flowing to the respective heating members between a heating drive state and a temperature detection state; a circuit for converting temperature values from each of the heating members to voltage values and for detecting the voltage values, using electrical current that flows during the temperature detection state; an analog/digital conversion circuit for converting said voltage into a digital value; an adder for cumulatively adding digital values obtained by the digital conversion from the start of heating; a comparator for comparing the cumulative value obtained by said adder against a target print density value which is set in advance with respect to a given point on which printing is to be executed and sent from a superior device, to thereby determine which one is greater; and a circuit for stopping the heating drive of said heating member if the comparator detects that the target print density has been reached.
Further, according to the present invention, in order to perform appropriate heating control with respect to the color developing medium to thereby obtain print images of excellent image quality, the temperatures of heat generated by each of the heating elements in the heated thermal head are measured and the generated thermal energy is calculated again and again at constant time intervals, producing the result that the intended color development density is achieved at each of the color developing points of the color developing medium.
Another aspect of the present invention is directed to a method of printing comprising the steps of: providing a print control device comprising a thermal head having a set of minute heating members, each heating member serving as both a heating element and a temperature detector, and a drive circuit; supplying an electric current to drive the heating members; switching the electric current supplied to the heating members between a heating drive state and a temperature detection state; converting temperature values from each of the heating members to a voltage value; determining the voltage value based on the electric current flowing during the temperature detection state; converting the voltage value into a digital value; adding the digital values to obtain a cumulative value; comparing the cumulative value with a target print density value to determine which value is the greatest; and interrupting the supply of electric current to the heating members if the target print density has been reached.